Hard disk drives include a servo-controller driving a voice coil actuator to position a read-write head near a track on a rotating disk surface. The read-write head communicates with the servo-controller, providing feedback used to control the read-write head's positioning near the track. The read-write head is embedded in a slider and floats on a thin air bearing formed above the rotating disk surface.
The voice coil actuator traditionally positioned the slider and its read-write head over the rotating disk surface. The voice coil actuator typically includes a voice coil, that swings at least one actuator arm in response to the servo-controller. Each actuator arm includes at least one head gimbal assembly typically containing a read-write head embedded in a slider. The head gimbal assembly couples to the actuator arm in the voice coil actuator.
A hard disk drive may have one or more disks. Each of the disks may have up to two disk surfaces in use. Each disk surface in use has an associated slider, with the necessary actuator arm. Hard disk drives typically have only one voice coil actuator.
Today, the bandwidth of the servo-controller feedback loop, or servo bandwidth, is typically in the range of 1.1K Hz. Greater servo bandwidth increases the sensitivity of the servo-controller to drive the voice coil actuator to finer track positioning. Additionally, it decreases the time for the voice coil actuator to change track positions. However, increasing servo bandwidth is difficult, and has not significantly improved in years. As areal densities increase, the need to improve track positioning increases.
One answer to this need involves integrating a micro-actuator into each head gimbal assembly. These micro-actuators are devices typically built of piezoelectric composite materials, often including lead, zirconium, and tungsten. The piezoelectric effect generates a mechanical action through the application of electric power. The piezoelectric effect of the micro-actuator, acting through a lever between the slider and the actuator arm, moves the read-write head over the tracks of a rotating disk surface.
The micro-actuator is typically controlled by the servo-controller through one or two wires. Electrically stimulating the micro-actuator through the wires triggers mechanical motion due to the piezoelectric effect. The micro-actuator adds fine positioning capabilities to the voice coil actuator, that effectively extends the servo bandwidth. In the single wire approach, the servo-controller provides a DC (direct current) voltage to one of the two leads of the piezoelectric element. The other lead is tied to a shared ground. In the two wire approach, the servo-controller drives both leads of the piezoelectric element of the micro-actuator.
Micro-actuation is today in its industrial infancy. What is needed is a practical, reliable and cost effective mechanism coupling a slider to a piezoelectric device to form a micro-actuator for use in hard disk drives.